It has long been known that the flight of a golf ball is dramatically improved if depressions or "dimples" are impressed on the surface of the golfball sphere. Aerodynamic studies and fluid mechanics principles attribute this improvement to the fact that the surface roughness produced by the dimples create turbulence at the surface of the sphere and hence what is known as a turbulent boundary layer. This turbulent boundary layer decreases the aerodynamic drag of the ball, thus allowing it to travel much farther than a smooth ball.
With conventionally dimpled golf balls, the creation of a turbulent boundary layer is highly velocity dependent. This is illustrated in FIGS. 1-4, labeled as prior art, which consider the flow of air or fluid over the surface of a portion of a golf ball 20. FIG. 1 shows the cross section of a typical, spherically concave golf ball dimple 22 which would be on the surface of the golf ball 20. In FIG. 2, air 24 passes slowly over the dimple 22 of FIG. 1 in the direction as indicated by the arrows. The air 24 conforms to the shape of the dimple 22 at its surface and has insufficient velocity or direction change to create turbulence or vortices.
FIG. 3 is a view of the same dimple 22 with the air 24 passing over the surface at a high enough velocity such that the air 24 cannot conform to the shape of the dimple 22. Instead, the air 24 slams into the back wall of the dimple 22 and quickly changes direction. As it exits the dimple 22, the air 24 cannot quickly re-conform to the spherical surface 26 of the golf ball 20. This results in the generation of turbulence and vortices, and thus the creation of the turbulent boundary layer.
FIG. 4 is a view of the same dimple 22 with the air 24 passing over the dimple at an intermediate velocity. The air 24 cannot perfectly conform to the surface of the dimple 22, but is in much greater contact than the air in FIG. 3 where the velocity is higher. As the air 24 exits the dimple 22, its velocity is such that it soon re-conforms to the surface 26 of the golf ball 20. Since this is the case, a turbulent boundary layer cannot be maintained even though some turbulence is generated at the intersection of the trailing edge of the dimple and the surface of the sphere.
The number, size, shape, and depth of the dimples all have an influence on the amount of distance improvement a dimpled golf ball will exhibit. Specifically, as the depth, diameter, and number of the dimples is gradually increased, the frictional drag of the ball is increased by the surface roughness of the dimples, and the aerodynamic drag is decreased. Up to a certain point, the effect of the reduction in aerodynamic drag far exceeds the effect of the increase of the frictional drag, and the golf ball exhibits significant distance improvement. Once this point is reached, though, further increases in dimple volume results in decreasing distance performance. This is because there is an increase in the frictional drag and an increase in aerodynamic drag due to the thickness of the generated boundary layer.
Those skilled in the art of designing golf balls have long known that the ideal dimple for a golf ball would change its shape during the flight of the ball. The ball would have low surface roughness when the velocity was high and turbulence was easy to generate. The roughness would increase gradually as the velocity decreased so as to maintain a uniform boundary layer, and would again decrease gradually to lower surface roughness during the descent of the ball, when one of the drag components would tend to keep the ball in flight. Unfortunately, there is no existing technology which allows golf balls to have such a feature.
Many attempts have been made to simulate at least a portion of the aforementioned ideal dimple characteristics. While there have been some improvements, these have been very modest in nature.
For example, triangle- or hexagon-shaped dimples having sharp edges have been used on golf balls. While these sharp edges assist in generating vortices and turbulence, they are located at the surface of the sphere and arc hence in the airflow during the entire flight of the ball. Their effect must therefore be regulated so as not to produce too much turbulence early on in the flight, making them ineffectual during later portions of the flight.
Other dimple shapes have also been proposed. U.S. Pat. No. 5,470,076 to Cadorniga discloses providing dimples inside dimples, wherein each dimple includes an outer concentric portion having a shallow spherical concavity and an inner concentric portion having a deeper spherical concavity, but these offer no projections in the airstream for generating vortices. Also, U.S. Pat. No. 5,536,013 to Pocklington discloses a toroidal dimple with a center projection extending up to the surface of the sphere. Since this projection reaches the surface of the sphere, it suffers from the same problems as the sharp edged dimples described above.
Turning now to the prior art shown in FIG. 5, U.S. Pat. No. 4,877,252 to Shaw discloses pairs of normal sized dimples 28, 30 that overlap by as much as twenty percent. A single projection 32 below the level of the golf ball surface 26 is formed where the two dimples 28, 30 overlap. Theoretically, during flight at intermediate velocities, air strikes the projection 32, further helping to create a turbulent boundary layer. However, because the dimples 28, 30 overlap by no more than twenty percent, they form a large area on the surface of the golf ball whose width is at least 1.8 times the diameter of a single dimple. This can be seen by comparing the indicated diameter D of the dimple 22 in FIG. 1 to the indicated diameter (1.8D) of the overlapping dimples 28, 30 in FIG. 5. Aerodynamically, the overlapping dimples 28, 30 in FIG. 5 will behave approximately as two independent dimples with only a slight improvement in flight characteristics. This is because the projection 32 is so far from the edges of the dimples 28, 30 that the air passing over the golf ball during flight will still have a chance to conform to the shape of the dimples even at relatively high velocities, e.g., as shown in FIG. 4.
U.S. Pat. No. 4,960,282, also to Shaw, discloses pairs or chains of dimples that preferably overlap one another by at least 0.02 inches (0.508 mm) or twenty percent. Although this disclosed structure potentially reduces the velocity at which a turbulent boundary layer is formed, it still does not provide enhanced flight characteristics at lower velocities. This is because the projection is still quite far from the edges of the dimples, and because the turbulent boundary layer producing effect of the overlapping pairs of dimples is highly directionally dependent. That is, with reference to FIG. 5, when air 24 flows in either of the directions indicated by the arrows, a turbulent boundary layer will potentially be formed, depending on the velocity of the golf ball 20 and the particular dimensions of the overlapping dimples. However, if the air flows along (instead of across) the projection 32 (e.g., normal to FIG. 5), no boundary layer effects will be produced.
Accordingly, it is a primary object of the present invention to produce a golf ball with unique dimples that overcomes the deficiencies of the prior art to increase the flight of the ball.
Another object is to provide golf ball dimples having a common cross-sectional structure wherein a turbulent boundary layer is formed at low, medium, and high velocities.
Yet another object is to provide golf ball dimples wherein the creation of a turbulent boundary layer is not dependent upon the direction air flows over the dimples.
Still another object is to provide golf ball dimples wherein a turbulent boundary layer can be produced without a resultant increase in frictional drag.